The present invention relates to AC generators and, more particularly, to AC generators that are driven by mechanical rotational force exerted on a shaft of the AC generator, in order to provide either a constant frequency output from a variable speed input, an output having a frequency within a narrow range from a variable speed input, or a constant frequency output that is shifted in space and time from a constant speed input.
Many aircraft include AC starter/generator systems to supply relatively constant frequency AC power. Many of the AC starter/generator systems installed in aircraft include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG includes a rotor having permanent magnets mounted thereon, and a stator having a plurality of windings. When the PMG rotor rotates, the permanent magnets induce AC currents in PMG stator windings. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current to the exciter.
The exciter typically includes single-phase (e.g., DC) stator windings and multi-phase (e.g., three-phase) rotor windings. The DC current from the regulator or control device is supplied to exciter stator windings, and as the exciter rotor rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the exciter rotor rectify this three-phase AC current, and the resulting DC currents are provided to the main generator. The main generator additionally includes a rotor and a stator having single-phase (e.g., DC) and multi-phase (e.g., three-phase) windings, respectively. The DC currents from the rectifier circuits are supplied to the rotor windings. Thus, as the main generator rotor rotates, three phases of AC current are induced in main generator stator windings. This three-phase AC current can then be provided to a load such as aircraft electrical systems.
Many of these AC starter/generator systems are driven by variable speed prime movers. For example, many generators are driven by the aircraft engines, which may vary in rotational speed during operation. Thus, to ensure the AC generators supply relatively constant frequency AC power, many aircraft include a hydro-mechanical transmission, or other type of gear arrangement, that converts the variable engine speed to a relatively constant rotational speed.
Although the above-described configuration is generally safe, hydro-mechanical transmissions can be relatively large, heavy, and complex, and they may also exhibit relatively poor reliability. Each of these factors can lead to increased overall aircraft, fuel, and maintenance costs, and/or increased maintenance frequency, which can further lead to increased costs.
One solution, proposed by both U.S. Pat. No. 6,188,204 to Vithayathil et al and U.S. Pat. No. 7,064,455 to Lando, employs three-phase windings for both main generator rotor and exciter generator rotor in order to control the main generator rotating field frequency and output frequency. The starter/generator system described in this disclosure employs two-phase windings for exciter and main generator rotor which greatly reduces the complexity and provides more flexibility to the starter/generator construction.
As can be seen, there is a need for a system and method of supplying relatively constant frequency AC power from a generator that is driven by a variable speed prime mover. There is also a need for a system that is a relatively small, lightweight, less complex, and more reliable, as compared to current systems and methods, and that does not rely on specified numbers of exciter and main generator poles.